Fix gradient background invalidation when HTML size changes

third_party/WebKit/LayoutTests/platform/mac/paint/invalidation/line-flow-with-floats-9-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "paintInvalidations": [
         {
-          "object": "LayoutBlockFlow HTML",
-          "rect": [0, 0, 500, 600],
-          "reason": "forced by layout"
-        },
-        {
           "object": "LayoutBlockFlow P",
           "rect": [8, 569, 418, 21],
           "reason": "incremental"
         },
         {
           "object": "LayoutText #text",
           "rect": [14, 242, 406, 126],
           "reason": "forced by layout"
         },
         {
@@ skipping 91 matching lines @@
     },
     {
       "object": "VerticalScrollbar",
       "reason": "scroll"
     },
     {
       "object": "LayoutView #document",
       "reason": "incremental"
     },
     {
-      "object": "LayoutBlockFlow HTML",
-      "reason": "forced by layout"
-    },
-    {
       "object": "LayoutBlockFlow P",
       "reason": "incremental"
     },
     {
       "object": "RootInlineBox",
       "reason": "incremental"
     },
     {
       "object": "LayoutText #text",
       "reason": "forced by layout"
@@ skipping 178 matching lines @@
       "object": "InlineTextBox 'here; the great\n'",
       "reason": "forced by layout"
     },
     {
       "object": "InlineTextBox 'wonder is, that there\u2018s any one left alive!\u2019'",
       "reason": "forced by layout"
     }
   ]
 }